University of Nebraska Lincoln, USA
Title: Quantifying and correcting for clay content effects on soil water measurement by reflectometers
Biography: Daran Rudnick
Clay content could affect the calibration of electromagnetic soil water sensors including reflectometers. To investigate this effect further, three TDR315 and three CS655 reflectometers were installed in each of five soils with clay content ranging from 5 to 49%. As the soils were dried in a temperature-controlled room, sensor reported soil volumetric water content (θv) according to factory calibration was compared against reference θv determined by weighing the soils. Sensor-reported θv was similar to reference θv in the sand (root mean square difference (RMSD) <0.02 m3 m-3), but the discrepancy was larger for the clayey soils. An increase in clay content tended to cause TDR315 to underestimate low θv and tended to cause CS655 to overestimate θv curvilinearly. At the levels evaluated in this experiment, differences in clay content produced a larger effect than temperature (24 versus 35°C) and salinity (0 versus 3.09 g/L CaCl2) on the accuracy of the factory calibration for both sensors. Soil-specific empirical calibrations developed using quadratic regression fitted the experimental data very closely (R2>0.93) for both sensors. By estimating calibration coefficients based on clay content alone and then adjusting sensor-reported θv accordingly, RMSD from reference θv was approximately halved for both sensors. Applying the same procedure to independent literature data resulted in improvements in a soil with 39% clay content but deterioration in a soil with 17-18% clay content. A clay content correction can enhance the accuracy of some reflectometers and electromagnetic sensors for soil water measurement.